I don't think they wanted the buildings to fall. The plan was to severely damage the buildings, leaving the works in such a condition that the buildings would be condemned for occupancy. These would eventually be demolished and the Freedom Tower would be erected in its spot.
But the operation when tits up when the first building fell. Building seven? Moot. People for years thought building seven was wired with explosives, just waiting for Larry Silverstein to give the order: "pull it". In the afternoon, the building returned to the dust.
Just a thought.
Maybe they shot down flight 93. You ever think about that? But instead we have a hero in the cell phone guy. "Let's roll." He's a hero either way, confronting the hijackers bravely.